1. Field of the Invention
The present invention generally relates to a mobile communication system, and in particular, to a Discontinuous Reception (DRX) method and apparatus for minimizing power consumption by a User Equipment (UE) in a connected mode or connected state, referred to herein as a connected mode UE.
2. Description of the Related Art
A Universal Mobile Telecommunication Service (UMTS) system is a 3rd generation asynchronous mobile communication system that uses Wideband Code Division Multiple Access (CDMA) based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), both of which are European mobile communication systems.
The 3rd Generation Partnership Project (3GPP) is in charge of UMTS standardization, and Long Term Evolution (LTE) is now under discussion as a next generation mobile communication system of the UMTS system.
LTE is a technology for realizing high-speed packet-based communication having a data rate of a maximum of about 100 Megabits per second (Mbps). To this end, several schemes are under discussion including, for example, a scheme for reducing the number of nodes located in the communication path by simplifying a configuration of the network, and a scheme for maximally approximating wireless protocols to wireless channels.
FIG. 1 shows a DRX operation of a connected mode UE according to the prior art.
Shown in FIG. 1 is an example of a communication system to which Node B scheduling is applied. Therefore, the UE supports uplink packet services over transmission resources allocated depending on the scheduling information from the Node B. Further, the Node B determines a Modulation and Coding Scheme (MCS) according to the quality of the allocated channel, and supports reception gain of the UE by applying Hybrid Automatic Repeat reQuest (HARM).
Referring to FIG. 1, the UE is a UE in the mode (or state) where the UE is Radio Resource Control (RRC)-connected to an upper layer. To save power, the connected mode UE, while keeping a sleep mode for a cycle having a particular length determined in an upper layer, wakes up at timings 105, 115, 120 and 125, and checks a downlink control channel allocated from the Node B to determine whether there is any data to receive. At timings 105, 120 and 125, there is no data to receive. At timing 115, there is data to receive. When there is no data to receive, e.g. at timings 105, 120 and 125, the UE switches (transitions) to the sleep mode and keeps the sleep mode until the next wake-up timing.
However, when there is any data to receive, as at timing 115, the UE receives the data from the Node B in step 140, and performs a process in step 145 of sending a response signal (ACK/NACK) indicating reception success/failure of the data transmitted based on HARQ. Thereafter, when the Node B sends a signal indicating that there is no more data to transmit, the UE stops the data reception and switches back to the sleep mode.
For convenience, the timing at which the UE wakes up is referred to herein as wake-up timing, and the time interval between the wake-up timings is referred to herein as DRX cycle length 110. The connected mode UE reduces power consumption through the DRX operation.
The UE receives signaling information, based on which the UE can calculate the wake-up timing, from the Node B in the call setup process. The wake-up timing is generally calculated as shown in Equation (1).Wake-Up Timing=First Wake-Up Timing+n×DRX Cycle Length  (1)
The first wake-up timing is generally calculated using constant information, such as, for example, an identifier of the UE, and the DRX cycle length is set by the Node B according to types of services and is then notified to the UE.
In the DRX operation, when the length of the DRX cycle is set long, the connected mode UE that performs no data transmission/reception for the DRX cycle, may probably suffer from uplink synchronization loss.
For example, a UE employing Adaptive Modulation and Coding (AMC) should periodically report the channel quality (or channel status) of the UE to the Node B at a particular timing. A connected mode UE, operating in a discontinuous transmission/reception mode, periodically reports the channel quality using a particular channel at a particular timing before the wake-up timing. This conventional channel quality report is very inefficient in that the UE reports the channel quality even when there is no need for actual data reception.
Therefore, a need exists for a discontinuous transmission/reception operation to minimize power consumption of a connected mode UE supporting a next generation mobile communication system, thereby solving problems of the prior art.
In addition, a need exists for an efficient channel quality report procedure and uplink synchronization procedure of the connected mode UE, distinguishable from conventional ones, for a next generation mobile communication system.